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Bad idle...saga continues

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79' GS850G. Ok a number of months ago my bike was stalling at idle. Tore apart the carbs, two fuel mixture screws were broken in the hole. One I had to punch out increasing the hole size. Carbs were dipped, new gaskets, o-rings the works and synched when I got it all running. Resealed the air box, new rings for the intakes. Valve adjustment was done (4 were too tight). Timing done. Basically I did everything I could to try and get her running smoothly. She's back together and running but runs rough and the carbs backfire as well as the exhaust on start up only. The plugs show black carbon so it's running rich. The bike will be running and then all of a sudden the RPM's drop and she sputters and will either stall or finally catch herself and idle again. I've sprayed starter fluid around looking for an air leak with no real success. I have just took the airbox and carbs off again in an attempt to sort this out, again. Couple of question right off the mark. I am guessing my attempt to compensate for the increased fuel mixture hole on #2 carb failed so I am going to have to find another carb or a rack of carbs..soooo. I want to find the same carb exactly as I can then swap parts but if I can't what other carbs will work on this bike? Second question. When I took off the airbox the rubber boots are plyable but they all spin in the airbox a fair bit and one especially, enough that I am thinking it would be a real problem for an air leak. The part of the boot that conects to the carb is soft and plyable but on all four the part that goes into the airbox is rigid. Is that normal? and how much play should they have on the airbox? So I am looking for a new carb/carbs and if anyone has suggestions for my line of attack that fire away. Oh ya, I bought a new batter a couple of weeks ago but this morning it was almost dead and I last rode it a week ago. I am guessing I have a problem there as well.
 
Hi Mr. azr,

Your carbs are Mikuni VM26SS. They are used on a few bikes, with jetting changes. For a look at an airbox fixup, see Mr. Roostabunny's here: http://picasaweb.google.com/roostabunny/GS850AirboxFixup

If the boots between the carbs and airbox can spin, I suspect a lot of air leaking there. It sounds like you also have charging issues. This is a job for the Stator Papers.

Thank you for your indulgence,

BassCliff
 
Mr. Roostabunny's example for an airbox rebuild is the one I actually followed. Worked like a charm. I did everything but replace the boots. Guess I had better do that then. I was thinking the same thing about the Stator papers when I found the battery dead this morning. Now as far as strong ignition, I haven't checked any of that yet. I was hoping since the bike started I would be fine in that department. I'm not that great of a mechanic so I'm not sure how a weak ignition would effect my idle, or does it? I'm pretty sure it's detailed on here on how to check all that so that'll be on my list of to do's. I know the type of carbs my bike has and that they are the stock ones for this bike but I have seen other model VM carbs for sale on here and other places but I am not sure if they would work on my bike or not. Does anyone have advice about buying carbs that are not exactly what the bike was built with? I think I'll put in new boots first before I purchase carbs. I guess I had better have a good look at my intake boots aswell. They seemed soft enough and I put in new rings and all, but obviously the airbox ones are no good. And thanks for your help Basscliff I know you have answered many of my different threads, and to everyone else that takes the time, this really is a great site for that.
 
If the only thing wrong with the carbs is a slightly enlarged hole on the pilot fuel screw I wouldn't give up on them just yet. The normal setting for the screws is 1 turn, but you might want to try 3/4 on the ones that are enlarged. Also, set the air screws to 2 turns. The symptoms you describe don't sound like air leakage to me since you say the carbs are black. Is your fuel tank clean and free from rust? Sometimes rust from the tank fouls the float needles and makes the bike run rich despite the fact that you just cleaned the carbs.

While you are at it, make sure your spark is strong; new points and plugs and make sure each cylinder is getting a properly bright spark. The plug caps can go bad so verify each cylinder just to be safe.

Hope this helps and good luck.
 
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Yes that was my plan, to set the fuel mixture screw further in. I went from 5/8 to 1/8 with no luck. Still had the rough idle, bit of carb back fire, and what seemed like intermittent vacuum loss. I'm not going to completely give up on the carbs just yet. Since finding out how bad the airbox boots were I'll order those first and see what that does. I did look at the points and they looked in great shape, I put new spark plugs in. The gas tank appears to be clean, as far as I can see into it with a flash light, I did think of that after the carb dip. Just to save me some search time, how exactly do I verify each spark plug is getting a good spark and that each spark plug cap is ok? thanks..
 
Pull the plug out of the engine and stick it back into the plug cap. Lay plug on side of engine to create a ground path and then watch for spark while turning over the engine.

Regarding your fuel screws, 1/8 turn open is not enough. The fuel screws should be at around 3/4 to 1 turn open - maybe slightly less for those buggered carbs. Also, where are your air screws set? They should be roughly double what the fuel screws are.
 
arrrrrr

arrrrrr

So I put my new airbox boots in (once the new ones showed up I realized how bad my old ones were) and reset the fuel and air mixure screws (best idle method) and resynched the carbs.....the bike is no better. Still has hesitation when throttled from a stop (bad enough it will stall) and has this intermitant carb backfire, bad enough it will stall when at an idle...arrrrrr
 
I had kind of a similar situation with my '79 GS850. My problem was driving me crazy but it was an easy fix......it was the petcock. Try running it on noting but Pri.

I found this out kind of by accident. As you may have read don't let it sit around while engine is not running with the petcock on Pri as fuel will continously run into the carbs and flood them and fuel may run into the base. I'm sure someone can explain it more eloquently than i.

Even with bad settings for my fuel and air screws this allowed the bike to run and then i went back and adjusted the screw settings. Before i could not get it to run long enough to adjust the screws at high RPM.

I hope this makes some kind of sense.
 
I see carb O rings but did you replace the intake boot O rings ??
 
Yes the intake boot rings were replaced. I actually thought I may have over torqued the intakes causing the rings to not seal properly but I checked that as well today and they were fine. Good suggestion about the petcock. I will try that tomorrow. I am guessing then that if the bike runs better on the "Pri" setting then that means the petcock is gone?
 
Mine runs like nothing is wrong just by switching over the the Pri position, good for me. I've read here don't waste your time rebuilding the petcock, buy a new OEM one (somewhere around $60). I'm doing that now along with trying to figure out which method to use to clean the inside of the tank. It's not that bad but there is rust in it.

Hope it works for you!
 
Ok! have you verified the jets in the carb are what it calls for ??. Might also make sure the mechanical ign advance is working!!
 
Ok, I was shocked. I turned my petcock to 'pri' and she ran like a top!! thanks 'Taz'. I really couldn't beleive it when I heard it. No backfiring, no hesitation, throttled perfectly and idled even better. So I'm going to try and order a new Petcock from someone up here in Canada as the shipping from the states is killing me. I recently order $61 worth of parts from Motogrid which included $17 shipping only to have the UPS guy at my door ask for another $37!!! That's nuts.....tell me am I doing something wrong with the shipping? anyone else Canadian and has better shipping tips than that? I'll keep you post on the petcock.
 
Glad to hear it runs well! I felt kind of dumb when all i had to do is switch to the pri position. It only took me about 10 days to figure that one out.

i was way over thinking it as i was going crazy trying to figure out why the heck it wouldn't run for me after going thru the whole carb cleaning. Tore the carbs apart twice to no avail. I think i have the original petcock which is going to turn 30 years old this coming year. I wish other things last for 30 years.
 
fixed

fixed

So I go into the timing again and this time check the points with a multitester. Turns out I am getting power to both points at the same time. So there is a short somewhere. I trace it as far as the harness going towards the tach/speedo and figure this could get ugly so I just unplug the two wires white and black and yellow and hook them directly to the coils by splicing them with the two orange wire from the coils and WHAMO!!! no back firing, no bogging, she ran and pulled like I've never seen her before. If I ever get ambitious I will trace down the actual short..maybe.
 
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